This invention relates to an optical coupled type isolation circuit which is used to transmit an analog signal with high accuracy from an input part to an output part in a state that the input part and the output part are electrically isolated from each other. In particular, this invention relates to an optical coupled type isolation circuit capable of reliable transmission of a one-bit data signal together with a clock signal easily.
An existing isolation circuit, e.g. an isolation amplifier, is used in various places, such as a factory, a plant, a hospital providing medical instruments, or the like to remove large common mode noises and/or to secure safety. The large common node noises are frequently caused at a place, e.g. the factory or the plant, where high power apparatuses and high sensitive electronic devices, which have different source voltages of over 1000 volts, are arranged close to one another.
A type of the isolation amplifier using a photo coupler transmits a signal in a form of light and thereby obtains electrical isolation between input and output parts thereof. Accordingly, the type of the isolation amplifier using the photo coupler is superior to a type of the isolation amplifier using an electromagnetic coupling in resistance to noises.
The isolation amplifier using the photo coupler, for example, is provided between a motor (or an AC servo, or an inverter for the motor) and a precision instrument, such as a microcomputer as a controller for the motor, in the factory or the plant to control the motor with high accuracy.
To improve accuracy and to reduce a cost of the isolation amplifier, a sigma-delta modulation is employed in an optical isolation amplifier. However, the optical isolation amplifier can not transmit a clock signal from an input part to an output part thereof, even if the clock signal is necessary in the output part.
Such an optical isolation amplifier is disclosed in U.S. Pat. No. 5,287,107 or Japanese Patent Publication No. 3174200.
As another exiting isolation circuit, there is a digital-analog (D/A) converter which adopts a pulse width modulation method.
The D/A converter converts input digital data into a pulse width modulation signal. The pulse width modulation signal is transmitted from an input part to an output part through a photo coupler. In the output part, the pulse width modulation signal is converted into an analog signal.
However, the D/A converter has a problem that jitters are caused to rising edges and falling edges of a light signal in the photo coupler and superimposed on the analog signal.
Such a D/A converter disclosed in Unexamined Japanese Patent Publication No. 6-209261.